Method for producing die-cast parts and a die casting device

ABSTRACT

The invention relates to a method for producing die-cast parts, in particular wheel rims or wheels for vehicles, in addition to a diecasting device therefor. To produce high quality wheel rims or wheels, the starting materials are heated in stages in a casting cell and are transferred in a semi-solid form to a die casting machine, where they are formed into die-cast parts.

The present invention relates to a process for manufacturing pressure die-cast parts, in particular parts made of aluminium or magnesium materials. The invention relates in particular to manufacturing wheel rims or wheels for the automobile industry.

Wheel rims or wheels for vehicles, in particular power vehicles, are these days manufactured by to the low-pressure casting process out of aluminium materials. This mostly involves simply controlled low-pressure casting machines with a high manual work component. Each machine requires an operator, who is exposed to both the high temperatures of the metal smelting as well as exhaust gases from the casting process. Mould release of the cast parts is expensive.

Many manual operations influence quality of parts and the cycle time for exhaust gas of a wheel is 7 to 12 minutes. Design modifications require new moulds to be made.

The manufacturer (low-pressure caster) buys the required aluminium alloy cast in bar form and melts these in smelting kilns near the low-pressure casting machine, that is, the quality may also deviate from machine to machine.

A process for casting vehicle wheels from aluminium, magnesium or similar light metals in low-pressure casting machines, which essentially comprise a crucible for warming the smelt, a casting mould arranged above the crucible as well as a feed pipe exiting from the casting mould, protruding through the ceiling of the crucible and penetrating into the smelt, whereby the metal used by casting the cast part is recharged to the casting machine, is disclosed by DE-C-3619525. It is characterised in that before each casting a metal block (pig) having a mass substantially corresponding to the cast weight is introduced to the gas chamber above the level of the casting machine and is introduced to the bath of the casting machine before or after the cast item is withdrawn or ejected.

It is generally known to manufacture parts out of aluminium or magnesium materials by pressure die-casting from the smelt or also semi-solid, trimmed billets (SSM process or thixomoulding). To date there has been no success in developing scientifically alternative processes for such highly charged parts.

DE-C-195 38 243 accordingly discloses a process for manufacturing a semi-fused thixomoulding casting material, wherein thixomoulding material, including an outer layer section with dendrite around an outer periphery of a main body section, undergoes heat treatment to produce a semi-fused casting material with coexisting solid and volatile phases. The dendrite, respectively dendrite fragments are transformed into spherical solid phases by raising the temperature of the outer layer section with respect to the main body section, and the outer layer section is brought to a semi-fused state. Excessively rapid heating of the main body is prevented by the use of skin effects. Such a gradual process for heating is expensive and requires several induction heating steps with different frequencies (capacities).

The object of the invention therefore is to develop a process for manufacturing pressure die-cast parts, in particular wheels or wheel rims for vehicles, by which the disadvantages of the prior art can be overcome. In particular, qualitatively high-quality wheels and the like should be able to be manufactured at reduced cost. The environmental burden should also be noticeably reduced, in conjunction with improved working conditions.

This task is solved by the features of Claim 1. Advantageous embodiments are disclosed in the sub-claims.

A further task of the invention is to create a casting device for carrying the process into effect.

Integrated into this type of known pressure die-cast cell with feed and warming devices for the casting material and a pressure die casting machine, handling apparatus and control devices are heating devices which enable gradual heating of billets made of aluminium materials (optionally also made of magnesium materials), which then enter the pressure die casting machine without significant contact with the ambient air. Following pressure die-casting or pressure conversion the cast parts are cooled in a special process and stored and certainly faster than previously was the case. The characteristic of the heating is that it begins with a high, constant energy input and the energy input is then reduced after the thixotropic texture is reached in the metallic area.

The essential advantages of the process according to the present invention are that it can be executed fully automatically; handling of volatile metal is omitted, including the corresponding melting furnace and potential dangers. Exhaust gases etc. do not have direct contact with operating personnel and the environment. The cycle time per wheel drops to ca. 1 minute.

The invention will be described in greater detail hereinbelow in an embodiment with reference to a diagram, in which:

FIG. 1: illustrates a casting cell

FIG. 2: illustrates a solid holding block in perspective view

FIG. 3: illustrates a holding block in partial section.

The casting cell contains a pressure die casting machine 1 with a mould, a converter for the heating plant 2, a monitor 3, a removal robot 5 for removals of items with a control unit 5, a loading robot 6 for feeding in slugs with control unit 7 as well as a spray robot 8 with control unit 9. Likewise provided are devices for mould tempering 10, cool stations 11 and 13, electrical, e.g. inductive heating units 12 for heating slugs (billets), a mechanism for part control before a conveyor belt 16 for removing substandard parts, as well as a further conveyor belt 15 for removing the cast parts. Also provided are a metal feed (slug bearing 17), an automatic cleaning station 18 as well as a manual cleaning station 20 for cast items, scales 19 and a repair area 21.

The pressure die-casting machine 1 is supplied directly with slugs or billets or portions, previously warmed in the heating units 12. They can be transferred directly and under airtight seal to the casting chamber of the pressure die casting machine 1. When magnesium materials are used (but also with aluminium materials) the warmed slugs are advanced by means of robots 6 in sealed containers to the casting chamber of the pressure die-casting machine 1. The containers are closed by a cover, so that the slugs are already heated in these and can be fed from the heating unit 12 to the casting chamber without inert gas when the set temperature is reached. The container is provided with a cover, which can be lifted, or which is hinged on the container. The cover or the floor of the container can also be provided with an additional and closable opening for pressurised or vacuum emptying for preparing molten portions. The container can be designed flat or high and narrow. Complete adaptation of the container to the shape of the material portion is not required, although the least possible volume of air in the closed container is an advantage. In particular, there should be only a minimal air gap between cover and material surface (portion). Sealing by moulding and intrinsic weight of the cover mostly suffices. In the case of semi-solid materials the container is simply emptied over the container edge. Alternatively, the billets can also be fed directly from a heatable preparation space/feed chute, which can also be optionally connected directly with the casting chamber, whereby the temperature can still be raised during transfer as required. In the case of aluminium materials e.g. an alloy No. A357 (AISi₇Mg 0.6) is preferably put to use, which enables quenching and storing of cast parts with minimal time expenditure (T5 treatment). Heating prior to casting takes place in stages and can be divided into three areas. High-capacity warming at the beginning, cooking after the thixotropic texture in the mantel region of the billet is reached and homogenising for final adjusting of globulithic texture in the pre-material.

Here an AI billet of 5 to 6″ diameter is first heated with constantly high energy to ca. 550 to 570° C., whereby the difference in temperature inside the billet is ca. 20° C. Next the energy input is severely reduced in a very short time, maintained over a very short time (cooking) and raised again thereafter and maintained, until a temperature of ca. 585° C. is reached and the temperature difference has dropped to ca. 2 to 3°. With billets up to 4″ diameter the procedural sequence can be simplified to the extent that the step of lowering the energy input can be carried out uniformly more slowly and over a longer period.

The individual billets can also be packed in foil prior to heating, preferably in aluminium foil, to prevent oxidation etc. Likewise, inert gas can be used to prevent oxidation.

A homogeneously distributed fine-grained material, which via the heating process forms an evenly distributed, globular microstructure with fineness of grain of ca. 50-100 μm is taken into consideration as a starting material for the slugs. The latter is necessary for e.g. qualitatively high-quality wheels or wheel rims, which is to be manufactured according to a SSM process.

There is no longer any volatile substance to handle. All exhaust gases are suctioned off and mould spraying is optimised such that no residual spray agent has to be disposed of. Generation of flue gas is practically excluded.

The moulds and casting tools are designed modular, so that there are cost-effective and easily exchangeable mould packets per wheel type. Quick exchange for each moulding part of the moulds is possible.

The mould in the pressure die casting machine 1 is e.g. two-part or three-part and can be employed in a variety of ways. Because the wheel rim bed remains unchanged depending on wheel dimension, only two mould plates need to be adapted to a different front design. These can still contain charges which comprise a favourable and less high-strength material. In this way the shelf life of such charges can be adapted to the required moulding cycle and only the base mould must comprise high-strength material.

Furthermore, the mould plates can contain slide inserts, which in defined regions permit adjustable heights and widths in the front design (depending on tyre and wheel rim dimension), so that not for every dimension is an extra mould required.

Special strippers enable easy mould release, even with complicated moulding or undercuts. These strippers are arranged such that after a mould opening stroke an upwards ejection stroke is made, whereby contoured parts/sliders travel transversely to the direction of mould release and can be detached from the cast item.

In accordance with a further embodiment the casting mould for manufacturing a vehicle wheel from e.g. aluminium materials has a fixed holding block 30 with mould centring 31 as well as mould parts or sliders 32 and cooling coils. The solid holding block 30 is connected in the standard manner to the casting chamber 34. It also has part-specific core elements for bores in the hub 36 of the wheel 35, which at the same time serve as ejection elements of the wheel 35. Accordingly no additional ejectors are required, though functional parts are used which are already needed for other tasks (here as core element). Filling the casting mould starts from the hub 36 of the wheel 35, and arranged on the hub 36 supported between the hub surface and the cutting face 37 of the casting chamber 34 as an insert is a filter or screen 38, which primarily serves to retain oxide skins of the SSM billet to be pressed in, or to tear these open and comminute them. The screen 38 is a part punched out of aluminium or steel and perforated to correspond to the hub surface. The billets, not illustrated here, are heated up to where they are O₂-deficient and then advanced. The mould cavity is filled in dry as far as possible, that is, without the use of lubricants. Due to the screen a shearing knife for the oxide skin of the billet can be dispensed with.

Casting chamber 34 and casting piston of the pressure die casting machine 1 preferably comprise a ceramic material in the attempt to dispense with lubricants and sprays as far as possible.

Compared to the previous direction of casting from the clearance surface of the wheel 35 substantially less material return occurs and the hub itself becomes discard. The residue of the screen 38 is removed along with the machining of the hub surface (if the screen 38 is made of steel, the aluminium-steel shavings can be used e.g. in steel production). The clearance surface of the wheel 35 is already cast true to end contours.

The cast parts or sliders 32 are fitted with drives and guides, not illustrated here, to ensure that the mould parts or slides 32 travel in and out.

Provided virtually perpendicular to the bearing plane of the solid holding block 30 are surfaces for locking the slider 32. There are also surfaces for locking the slider 32 provided which are arranged parallel to the abovementioned bearing plane on the slider 32 or the cast parts. Likewise, other surfaces for combined counter-locking and mould centring are provided virtually perpendicular to the abovementioned bearing plane.

Corresponding counter-contours are designed in the mobile holding block 33.

The faces for locking the slider 32 are located preferably on or in the solid holding block 30 and particularly preferably in a line. The faces for mould centring circle the mould independently of the number of mould parts or sliders 32.

The forces acting during casting are thus distributed in the solid holding block 30 onto the faces for combined mould centring and counter-locking right and left by the slider 32—introduction of force and forwarding of force are thus on a line and are more easily controlled.

The mould parts or sliders 32 are available for the area of wheel sizes (e.g. from 13″ to 22″) and can be arranged in only one set of holding blocks 30, 33.

Legend

-   1 pressure die-casting machine -   2 converter -   3 monitor -   4 removal robot -   5 control robot -   6 loading robot -   7 control unit -   8 spray robot -   9 control unit -   10 mould tempering -   11 cooling station -   12 heating unit -   13 cooling station -   14 part control -   15 conveyor belt -   16 conveyor belt -   17 slug bearing -   18 cleaning station -   19 scales -   20 cleaning station -   21 repair area -   30 solid holding block -   31 guide -   32 slider -   33 mobile holding block -   34 casting chamber -   35 wheel -   36 hub -   37 cutting face -   38 screen 

1. A process for manufacturing pressure die-cast parts, in particular wheels or wheel rims for vehicles by casting, whereby the starting materials are supplied and heated in the form of billets or slugs, whereby the heated starting materials have a globulithic microstructure, are fed in semi-solid form to a pressure die casting machine (1) and can be cast or shaped at high pressures, characterised in that the heated starting materials are supplied to the pressure die casting machine (1) with extensive exclusion of air, and the cycle time is ca. 1 minute.
 2. The process as claimed in claim 1, wherein the starting materials are heated in stages.
 3. The process as claimed in claim 1, wherein cooling is carried out at least at 180° C. and storage of the cast parts can be shortened (T5).
 4. The process as claimed in claim 2, wherein the billet made of an aluminium material is first heated at a high and constant energy input to a temperature, at which a thixotropic texture is set in the mantel area, after which the energy input is drastically and quickly reduced and maintained thus for a short time, after which the energy input is again raised and maintained until the temperature inside and in the mantel area is extensively adjusted.
 5. A casting device for manufacturing pressure die-cast parts, in particular wheel rims or wheels for vehicles with a casting device, heating devices for heating the starting materials and handling apparatus, in particular robots for handling the starting materials and pressure cast parts, wherein the casting device has a pressure die casting machine and is designed as an automated casting cell, which exhibits variable forms and charges of the forms, whereby ejector elements are provided, which at the same time are functional elements (during the casting procedure).
 6. The casting process as claimed in claim 5, wherein the material of a charge is adapted to the required moulding cycle.
 7. The casting device as claimed in claim 5, wherein the mould includes a solid and a mobile holding block (1, 4) and mould parts or sliders (3), which are designed modular, arranged therein in guides.
 8. The casting device as claimed in claim 5, wherein the mould parts or slider (3) can be placed for specific parts sizes in the holding block (1, 4).
 9. The casting device as claimed in claim 1, wherein casting chamber and piston of the pressure die casting machine (1) comprise a ceramic material.
 10. The casting device as claimed in claim 5, wherein casting chamber and piston of the pressure die casting machine (1) comprise a ceramic material. 